Such transmissions are usually reversible and the driving element may become the driven element and vice versa depending on the external circumstances, but for descriptive convenience it is assumed hereinafter that the element fixed to the planet carrier is the driving element.
Briefly stated, such transmissions operate as follows. The driving element is rotated by an external power source and it causes the planet carrier to rotate about the transmission's stationary axis. The planet carrier therefore causes the planet wheel to orbit with the orbiting axis, that is to say causes the planet wheel to roll around the stationary ring member, which causes the planet wheel to rotate about its own (the orbiting) axis. The planet wheel is engaged with the sun wheel and therefore the rotation of the planet wheel causes the sun wheel to rotate to a greater or lesser extent, depending on the extent to which the orbital movement of the planet wheel about the sun wheel augments or diminishes the effect of its rotation on that wheel. The rotation of the sun wheel induced by the combined effect on it of the orbital movement and rotation of the planet wheel is transmitted to the driven element.
The velocity ratio of such a transmission is determined by the relative values of the diameters of the ring member, planet wheel and sun wheel. Thus if the diameters of the planet wheel and sun wheel are left unchanged, the velocity ratio could be varied by altering the diameter of the ring member. Indeed proposals to provide variable ratio epicyclic or similar transmissions wherein the ring member is of variable diameter have been put forward hitherto. One such is the transmission described and illustrated in the International patent application PCT/AU/89/00241 published under publication No. WO 89/12185.